Conventional query processing systems often make use of Standard Industrial Classification (SIC) codes or other similar codes which identify categories of products and services. For example, such a system may process a user request for information on a particular product or service in order to determine the corresponding SIC code, and then use the SIC code to retrieve and display the appropriate information to the user. SIC codes are organized around families of relationships in the form of a parent-child hierarchy. The relationships between items in different levels of the hierarchy are generally in terms of supersets and subsets. FIG. 1 illustrates a conventional SIC parent-child hierarchy 10 which includes SIC code 3711-02 for Automobile Manufacturers. The hierarchy 10 includes SIC code 3700 for Transportation Equipment Manufacturers, and a number of subsets of SIC code 3700, including a subset 12-1 corresponding to SIC code 3711 for Motor Vehicle and Passenger Card Bodies, a subset 12-2 corresponding to SIC code 3713 for Trucks and Buses, as well as other subsets. Although the hierarchy 10 of FIG. 1 can provide a great deal of information regarding different types of manufacturers, including automobile manufacturers, it does not provide information on other products or services which are not within the hierarchy.
A known technique for expanding the amount of relevant information provided in response to a given user query makes use of keyword relationships between SIC code entries in different hierarchies. This technique may utilize a keyword search across multiple hierarchies in order to provide the user with additional relevant information. For example, a user query regarding automobile manufacturers may be processed to extract the keyword "automobile." Any SIC code entry having that keyword or a sufficiently similar version thereof in its descriptor may then be retrieved and displayed to the user, in addition to the entry for automobile manufacturers. These keyword-related SIC codes may include, for example, SIC code 5500 for Auto Dealers and Service Stations, SIC codes 6141-03 and 6141-04 for Loans and Financing Automotive, and SIC code 6411-11 for Insurance Automotive. Although this keyword-based technique provides an improvement over simply retrieving a single SIC hierarchy, it fails in many situations to provide the user with the most relevant and complete information. For example, there may be many categories of goods and services related to an SIC code obtained from a user query which the keyword-based technique will fail to retrieve. This inability to provide the user with information from otherwise unrelated multiple hierarchies is a significant drawback of conventional query processing systems.